The present invention relates, in general, to information storage, and more particularly to storage of semiconductor wafer map information.
Semiconductor devices are typically fabricated as a plurality of individual die on a single wafer. A single wafer may contain as many as ten thousand discrete transistors or diodes. During the manufacture of these devices each individual die is typically tested before separation from the wafer. This testing comprises visual and electrical tests designed to distinguish unusable die from good die. To facilitate separation of the good and bad die, the bad die are typically marked using a colored ink dot. A common refinement of this testing uses the test results to separate individual die into categories. For example, each different category may each have a specific voltage range, temperature range or speed range. Typically each category is then marked with a different color ink dot. After the wafer is diced into individual die, the desired die are selected based on the coding of the ink dot. Likewise a plurality of different types of device may be fabricated on a single wafer to be separated after dicing by means of the ink dot marking.
Marking the die with ink dots has definite shortcomings. The number of useable categories is limited by the number of easily distinguished ink colors available. The ink can chemically damage the die or the packaged device. The ink can smear or run from one die to another die or the dot can dry and fall off the die. Any of these produces incorrectly marked die and causes incorrect die to be selected. The solvents which must be used to remove the ink for cleaning equipment and from marked die are hazardous chemicals which are difficult to use and store. Handling of the inks themselves is messy and requires extra housekeeping effort as well as causing ink stains on equipment, personnel and their clothing. There has long been perceived a need to eliminate the wafer inking process from semiconductor manufacturing. As a result, many alternative methods of recording the results of the wafer test information without using ink have been developed.
One such recording method uses a computer to generate a wafer map in memory which shows the category assigned to each die on the wafer as a numerical value. This wafer map may then be printed on paper or transferred electronically to the final assembly location. This wafer map information is then used to select the desired die as before. This method eliminates the use of ink and associated solvents. However many errors are introduced during transfer of the wafer and the corresponding wafer map information. The wafer map may be swapped with the wafer map from a different wafer, the wafer map may be incorrectly oriented with the wafer, the wafer map damaged, or the wafer map may be lost entirely. Similarly the wafer itself may break and die selected from a plurality of partial wafers. Each of these partial wafers must be handled separately and requires a copy of the correct part of the wafer map. These problems are often aggravated since the location where testing is performed is often a great distance from the location where dicing, die selection and final assembly takes place. As a result detection of such errors and subsequent correction becomes extremely difficult.
Another method which addresses this problem is described in U.S. Pat. No. 4,510,673, entitled "Laser Written Chip Identification Method", issued Apr. 16, 1985 to A. J. Shils et al, assigned to International Business Machines Corp., and which is incorporated herein by reference. Shils' method uses a laser to mark the back of each die with a unique identifying code. This method is oriented to use of an inverted mounting of the die in which the back surface of the die is exposed. Once written in this way, the code is read manually using low power magnification. This code is then used to verify the selection of the correct die and placement of the die on a multi-chip substrate. This approach allows the identification code to be used throughout the life of the die for assembly, verification, system test, and defect analysis. However Shils' method still requires matching of an external database containing information about the die with the die itself. Since the entire area of the top surface of the die is used to fabricate active devices, Shils' method can only be used on the back of the die. The most commonly used die mounting method however supports the die by the back of the die rather than by raised areas on the active surface of the die as taught by Shils. Thus the unique code marked on the back of the die cannot be read without destroying the packaged device. Shils' method also cannot be used with the typical die selection and final assembly process since the handling equipment is designed to hold and support the die by the back, obscuring the code.
There is a need for a way to record the die selection information in such a way that the information cannot be separated from the wafer. The information must not be corrupted or lost even if the wafer itself is damaged. The method should allow for breakage of the wafer into smaller portions, and should not use chemical marking methods. The method should be compatible with automated assembly techniques and with typical die assembly methods.